1. Field of the Invention
The present invention relates to variants of a glycoside hydrolase having one or more improved properties relative to its parent enzyme, nucleic acids encoding the variants, methods of producing the variants, and methods of using the variants.
2. Description of the Related Art
Cellulose is a polymer of the simple sugar glucose covalently bonded by beta-1,4-linkages. Many microorganisms produce enzymes that hydrolyze beta-linked glucans. These enzymes include endoglucanases, cellobiohydrolases, and beta-glucosidases. Endoglucanases digest the cellulose polymer at random locations, opening it to attack by cellobiohydrolases. Cellobiohydrolases sequentially release molecules of cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble beta-1,4-linked dimer of glucose. Beta-glucosidases hydrolyze cellobiose to glucose.
The conversion of cellulosic feedstocks into ethanol has the advantages of the ready availability of large amounts of feedstock, the desirability of avoiding burning or land filling the materials, and the cleanliness of the ethanol fuel. Wood, agricultural residues, herbaceous crops, and municipal solid wastes have been considered as feedstocks for ethanol production. These materials primarily consist of cellulose, hemicellulose, and lignin. Once the cellulose is converted to glucose, the glucose is easily fermented by yeast into ethanol.
Ståhlberg et al., 1996, J. Mol. Biol. 264: 337-349, describe activity studies and crystal structures of catalytically deficient mutants of cellobiohydrolase I from Trichoderma reesei. Boer and Koivula, 2003, Eur. J. Biochem. 270: 841-848, disclose the relationship between thermal stability and pH optimum studied with wild-type and mutant Trichoderma reesei cellobiohydrolase Cel7A.
WO 2004/016760 discloses variants of a Hypocrea jecorina cellobiohydrolase.
It would be an advantage in the art to provide glycoside hydrolase variants with improved properties for converting cellulosic materials to monosaccharides, disaccharides, and polysaccharides. Improved properties include altered temperature-dependent activity profiles, thermostability, pH activity, pH stability, substrate specificity, product specificity, and chemical stability.
It is an object of the present invention to provide variants of glycoside hydrolases with improved properties compared to its parent enzyme.